🚀 go-pugleaf

In article <1qmla6INNb25@fmsrl7.srl.ford.com> hugh@slee01.srl.ford.com writes:
|I wrote:
|: We have an application here that does a lot of floating point
|: calculations. We have noticed differences in results between gcc and
|: other compilers. i.e. cc on Sun, Borland C on PC, and MS C on PC give
|: identical results. Only gcc on Sun differs. Does gcc do something
|
|I think I know the problem. It has to do with differences in the
|treatment of floats in traditional and ANSI C.
|
	...
|Here is the epsilon output run on Sun with three different
|compiler/switch combinations:
|
|    slee06% cc -o epsilon epsilon.c
|    slee06% epsilon
|    Epsilon machine (avec variable intermediaire) : 1.19209e-07
|    Epsilon machine (sans variable intermediaire) : 2.22045e-16
|
|    slee06% gcc -o epsilon epsilon.c
|    slee06% epsilon
|    Epsilon machine (avec variable intermediaire) : 1.19209e-07
|    Epsilon machine (sans variable intermediaire) : 1.19209e-07
|
|    slee06% gcc -traditional -o epsilon epsilon.c
|    slee06% epsilon
|    Epsilon machine (avec variable intermediaire) : 1.19209e-07
|    Epsilon machine (sans variable intermediaire) : 2.22045e-16
|
|The epsilon values from gcc's float.h are:
|
|    #define FLT_EPSILON ((float)1.19209290e-07)
|    #define DBL_EPSILON 2.2204460492503131e-16
|
|As expected, cc appears to be doing double precision arithmetic. The
|intermediate variable is truncating to float. The first run of gcc has
|identical results with or without the intermediate variable. This
|indicates it is not promoting float to double for arithmetic
|operations. The second run of gcc, with the -traditional compiler
|flag, gives the same result as for cc. Even though the gcc manual page
|gives no indication of this, -traditional must be causing gcc to treat
|float arithmetic the same way as cc does.
|
|The Borland compiler gave
|
|    Epsilon machine (avec variable intermediaire) : 1.19209e-07
|    Epsilon machine (sans variable intermediaire) : 1.0842e-19
|
|Summing things up, it looks like everybody but gcc is promoting floats
|to double.
|
|We tried compiling our application with gcc -traditional and the
|results matched those of cc.

	Sun's Solaris 2.1 SPARCompilers 2.0.1
gives:
oceanus> cc epsilon.c ; a.out
Epsilon machine (avec variable intermediaire) : 1.19209e-07
Epsilon machine (sans variable intermediaire) : 2.22045e-16
oceanus> cc -Xs epsilon.c ; a.out
Epsilon machine (avec variable intermediaire) : 1.19209e-07
Epsilon machine (sans variable intermediaire) : 2.22045e-16
oceanus> cc -Xa epsilon.c ; a.out
Epsilon machine (avec variable intermediaire) : 1.19209e-07
Epsilon machine (sans variable intermediaire) : 1.19209e-07

So it seems gcc is entirely consistant with other C compilers,
albeit it's default is ANSI, where as other compilers default
to K&R. (in regards to float promotion)


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